In order to obtain various printed matters such as printed matters for use in office form, printed matters for various kinds of books, printed matters for various kinds of wrappings such as carton paper, various kinds of plastic printed matters, printed matters for use in seal or label, art printed matters, and metal printed matters (art printed matters, printed matters for use in a can for drinking, and printed matters for use in food such as canned food), various kinds of printing methods such as a lithographic printing method (standard lithographic printing method that uses dampening water and water-less lithographic printing method that does not use dampening water), a letterpress printing method, an intaglio printing method, and a stencil printing method have been adopted, and inks suitable for the respective printing methods have been used to print. As one of such inks, an active energy ray-curable ink is known. An existing active energy ray-curable ink is cured by use of a light source such as a mercury lamp or a metal halide lamp, and a photopolymerization initiator matched to an emission wavelength used to cure the ink is used. UV-ray irradiation light sources mentioned above have an emission region over a wide wavelength region; accordingly, several kinds of photopolymerization initiators having different absorption wavelengths (see, patent document 1) are used in combination in existing active energy ray-curable inks corresponding to a color of ink or a kind of a light source used to cure. Thus, existing active energy ray-curable inks can respond to curing of an ink with a mercury lamp, a metal halide lamp or the like because it uses photopolymerization initiators having different absorption wavelengths. However, when an ink is cured with a monochromatic light source such as a UV-LED, the ink cannot be sufficiently cured. Furthermore, when the existing active energy ray-curable inks are cured with UV-LED, a photopolymerization initiator having an absorption in a wavelength from 350 to 420 nm has to be used. However, pigments used in inks have, though different in magnitude of absorption thereof, light absorption in a wavelength from 350 to 420 nm in many cases. Accordingly, even if a photopolymerization initiator having absorption in a wavelength from 350 to 420 nm is used as a photopolymerization initiator of an ink, the ink cannot be sufficiently cured. In particular, a black ink and a cyan ink has a large light absorption in the vicinity of a wavelength of 365 nm compared with a yellow ink and a magenta ink; accordingly, even if a photopolymerization initiator having absorption in a wavelength from 350 to 420 nm is used, curability thereof is poor since sufficient light energy cannot be imparted to the photopolymerization initiator. Usually, a multicolor overprinting with a black ink, a cyan ink, a magenta ink and a yellow ink is carried out in this order in process-printing. However, as a film thickness of inks becomes higher in a portion where color density is deep or color inks are overprinted, light transmittance decreases towards a depth direction. As a result, curability in the ink deteriorates. In particular, since the black ink and the cyan ink are cured by light passed through portions of the yellow ink and the magenta ink in a four-color overprinted portion, the internal curability is remarkably deteriorated. In this connection, as a method for imparting curability by an UV-LED, an ink composition using a triazine initiator has been proposed (see, patent document 2).
Patent document 1: Japanese Patent Laid-Open (JP-A) No. 6-157962
Patent document 2: JP-A No. 2007-23151